1. Field of the Invention
This invention relates to regenerative transmission systems for tracked vehicles, ie transmission systems in which power given up by the inner track when the vehicle turns is transferred to the outer track, of the type having
a rotatable power input member;
a change-speed gear unit coupled to the power input member;
a gear unit member rotatably drivable by the gear unit;
a steering differential having a rotatable steering input member coupled to the power input member and having a first and a second rotatable steering output member arranged so that the sum of their rotational speeds is proportional to the rotational speed of the steering input member;
a first and a second drive differential coupled to the gear unit output member and the first and second steering output member, respectively, and having a first and a second rotatable output member, respectively, the first and second drive differentials being configured such that the rotational speed of the first and second output members is equal to a linear combination of the rotational speeds of the gear unit output member and of the first and second steering output member, respectively; and
a first and a second steering brake associated with the respective steering output members.
Such a transmission system will be referred to hereinafter as "a transmission system of the type specified".
2. Description of the Prior Art
In known transmission systems of the type specified the steering differential is a spur gear differential the cage of which is directly geared to the power input member and the steering output members are half-shafts the sum of whose rotational speeds is thus a fixed multiple of the rotational speed of the input member. Such systems are known as triple differential transmission systems. When a vehicle fitted with a triple differential transmission system is moving in a straight line the output members, which drive the tracks of the vehicle, are rotated at the same speed made up of two components combined by the drive differentials. The components consist of the rotational speed of the gear unit output member and the rotational speed of one of the two steering differential half-shafts. To steer the vehicle one of the steering brakes is applied to bring the corresponding steering differential half-shaft to rest. Because the sum of the rotational speeds of the half-shafts is maintained at a constant value by the steering differential, assuming a fixed engine speed, the other half-shaft speeds up to rotate at twice its previous rate. The effect on the output shafts driving the tracks is to speed up one and slow down the other by the same amount, which amount is independant of the gear ratio selected by the change-speed gear unit.
Such transmission systems are well proven in service, relatively reliable and not unduly expensive to manufacture, compact, and efficient in power usage which make them attractive for use with heavy duty tracked vehicles such as tanks. However, it is a characteristic of such systems that although there is a different radius of turn for each gear ratio the maximum rate of turn, obtained when the vehicle engine is rotating the input member at the maximum speed, is constant for all the gear ratios. Consequently, as higher gear ratios are selected the maximum lateral acceleration experienced by the vehicle when it turns increases.
The safe maximum speed of the vehicle is the speed at which, in a turn, the maximum lateral acceleration becomes equal to the accepted skid out limit at which the adhesion of the track on the ground will fail.
For a vehicle with a low maximum speed the transmission system can be designed to have a relatively high value of maximum rate of turn thus providing the vehicle with high manoeuvrability at low speeds. However, if this known transmission system is used for a vehicle with a high maximum speed, such as a tank, it must have a relatively low maximum rate of turn to allow the vehicle to turn with safety at its top speed. This correspondingly decreases its low speed manoeuverability which is a serious disadvantage.